Microfluidic colloid filtration

a) Tortuous path of a small, fluorescent tracer particle. The image is a stack from 600 individual microscope images. The total travel time for the observed particle is 12.52 s.
b) Tracked path of the fluorescent tracer particle with regions of short range order I, II and III. Speed color coded path through the partly crystalline bed.

Filtration of natural and colloidal matter is an essential process in today’s water treatment processes. Membrane fouling is the most substantial problem in membrane filtration: colloidal and natural matter build-up leads to an increasing resistance and thus decreasing transport rate.We present a method to follow filter cake build up as well as transport phenomena occuring inside of the fouling layer. The microfluidic colloidal filtration methodology enables the study of complex colloidal jamming, crystallization processes as well as translocation at the single particle level.

Membrane filtration is the essential process in water purification, sterile filtration and bioprocessing. The greatest challenge in membrane filtration is to gain control over the separation process. While the rejection of matter is the essential function of a membrane, the rejected matter poses a continuously increasing resistance for the filtration process. This phenomenon is known as membrane fouling. In a normal filtration process, the interaction between particles and the membrane occurs over a very short initial time. This initial interaction may preordain the entire downstream deposition process. Very little is known about the integral phenomena occurring during layer deposition and its transport processes.Our model system composed of a microfluidic chip featuring a set of parallel microchannels with constrictions to mimic membrane pores aims at resolving this issue. To showcase the potential of this technique we can track small, hard and fluorescent particles as they enter the filter cake and translocate towards lower pressures following a tortuous path.We observe that the tracer particle moves faster in domains with short range order and is slowed down in amorphous region. The particle moves fast (orange) where the interstitial channels are straight and roughly aligned with the flow direction. This data leads us to the conclusion that particle translocation through a crystal is favoured over transport through amorphous regions, even though the pores are smaller than in the amorphous regions. This is counter intuitive as one would expect an irregular filter cake to transport matter faster as its packing density is lower, resulting in a higher volume flow rate.